An organic electroluminescence device (hereinafter, referred to also as an “organic EL device”) using an organic electroluminescence material can emit light of various colors in a visible light range in addition to white light when using an appropriate organic material or an appropriate structure. Therefore, display devices and illumination devices using an organic electroluminescence device are being progressively developed.
An organic electroluminescent display device includes an organic EL device in each of pixels. The organic EL device in each pixel is connected to a transistor, and light emission is controlled via the transistor. The organic EL device is formed over a device substrate having the transistor formed thereon. In a typical pixel structure, an interlayer insulating layer is provided between the organic EL device and the transistor. One of electrodes included in the organic EL device and a source electrode or a drain electrode of the transistor are electrically connected to each other in a contact hole formed in the interlayer insulating layer.
An organic electroluminescent display device includes a pixel array including a plurality of such pixels arrayed in a matrix. Light emitted by the organic EL device is emitted toward the device substrate or a counter substrate facing the device substrate, and thus a display screen is formed over a surface of the device substrate or the counter substrate.
The organic EL device includes a layer containing an organic electroluminescence material (hereinafter, referred to as an “organic electroluminescence layer” or an “organic EL layer”) between a pair of electrodes, which are also referred to as a positive electrode and a negative electrode. Light emitted by the organic EL layer expands at all the angles of 4π as represented by solid angle. Therefore, how to effectively use the light output in a direction other than toward the display screen is an issue of studies.
There is a problem that the light emitted and propagated parallel to a film surface of the organic EL layer is emitted toward an end surface of the organic EL layer and is not emitted from a transparent electrode side. A reason for this is that the material forming the organic EL layer has a high refractive index (n=1.8 to 1.9), and thus light incident on an interface between layers having different refractive indices at a specific angle of incidence is totally reflected. For example, light totally reflected at an interface between the organic EL layer and the transparent electrode or an interface between a glass substrate and air is guided in the organic EL layer or the glass substrate and is absorbed in the layer or is emitted from an end surface of the glass substrate. Light guided in the organic EL layer or the glass substrate in this manner is referred to as “waveguide light”.
Due to the light guided in the organic EL layer, the extraction efficiency of the light generated in the organic EL layer (ratio of the light emitted toward the glass substrate with respect to the total amount of light emitted by the organic EL layer) is considered as being about 20%. Increasing the extraction efficiency of the light generated in the organic EL layer is important to reduce the power consumption of the organic electroluminescent display device using the organic EL device.
Light that is emitted obliquely with respect to a surface of the pixel including the organic EL device is leakage light. The leakage light emitted from an adjacent pixel and causes a problem of color mixture. Therefore, usually, a light shielding layer is provided to partition the areas of adjacent pixels so that light emitted from one pixel is prevented from being mixed with light emitted from a pixel adjacent thereto. However, when the width of the light shielding layer is increased in order to reduce the influence of color mixture, a problem occurs that the numerical aperture of the pixels is decreased and the light emitted by the organic EL layer is not efficiently used.
In an organic electroluminescent display device disclosed in Japanese Laid-Open Patent Publication No. 2001-291588, an organic EL layer is formed even in a contact hole in which an electrode connected to a transistor and a pixel electrode are connected to each other, and an interlayer insulating layer is formed of a light-transmissive insulating material such as polyimide, polyamide, acrylic resin or the like. A side wall of the contact hole includes a steeply inclining surface. In such a part of the organic electroluminescent display device, usual light emitting luminance is not obtained, and in addition, the light emitted from pixels cannot be effectively used. The above-identified publication also discloses a structure in which the contact hole is buried with an organic resin so that an organic EL layer is not formed in this part.
An organic electroluminescent display device disclosed in Japanese Laid-Open Patent Publication No. 2003-229283 discloses a structure in which a peripheral portion of a pixel electrode is covered with a partition film (insulating film). The pixel electrode is connected to lines in an underlying layer provided below the pixel electrode through a contact hole formed in an insulating layer also formed below the pixel electrode. The partition film is provided over the contact hole and covering the same.
According to the conventional technologies, an organic EL device including an organic EL layer is mainly formed over a flat area. Therefore, as described above, light propagated in the organic EL layer in a horizontal direction (namely, waveguide light) cannot be used. In other words, no measure is taken to effectively use the light staying in the organic EL layer.